Interplay between Point Defects and Thermal Conductivity of

Article pubs.acs.org/JPCC

Interplay between Point Defects and Thermal Conductivity of Chemically Synthesized Bi2Te3 Nanocrystals Studied by Positron Annihilation H. F. He,† X. F. Li,† Z. Q. Chen,*,† Y. Zheng,‡ D. W. Yang,‡ and X. F. Tang*,‡ †

Hubei Nuclear Solid Physics Key Laboratory, Department of Physics, Wuhan University, Wuhan 430072, China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,Wuhan University of Technology, Wuhan 430070, China

‡

ABSTRACT: In this work, Bi2Te3 nanocrystals were synthesized via a hydrothermal method. They were treated by spark plasma sintering (SPS) at 350 °C and further annealed between 350 and 500 °C. The crystal structures and morphologies of these annealed Bi2Te3 samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) measurements. SEM observation indicates an obvious increase of particle size with increasing annealing temperature, but the grain size estimated from HRTEM observation and the broadening of X-ray diffraction lines show little change in the annealing temperature range between 350 and 500 °C. Positron annihilation lifetime measurements reveal vacancy defects in all of the samples, which exist most probably in the grain boundary region. The average positron lifetime shows a monotonic decrease from 301 to 273 ps with increasing annealing temperatures up to 500 °C. Detailed analysis of the positron lifetime indicates decrease of vacancy concentration after annealing. Meanwhile, the lattice thermal conductivity of the Bi2Te3 nanocrystals increases with increasing annealing temperature. The electrical resistivity and Seebeck coefficient have also some changes for the annealed samples. The intimate correlation between vacancy defects and lattice thermal conductivity confirms that reduction of thermal conductivity in Bi2Te3 nanocrystals is due to phonon scattering by vacancy defects rather than grain size effects.

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where Ω0, v, x, ΔM, and M are the volume of the unit cell, lattice sound velocity, fraction of the guest atom, atomic mass difference between the guest and host, and average mass of the cell, respectively. According to this equation, one can find that the larger is the mass difference, the larger will be the reduction in kL. The maximum atomic mass difference can be achieved by introducing vacancy-type defects on one or more lattice sites; therefore, the maximal reduction of lattice thermal conductivity can be achieved. Recently, there have been several reports about the influence of vacancy-type defects on thermal conductivity and figure of merit ZT. Pei et al. found that the lattice thermal conductivity had a large reduction by introducing In vacancies in In2Te3−InSb solid solutions.6 Kurosaki et al. also unexpectedly found that in Ga2Te3 bulk materials, phonon scattering by vacancy-type defect caused very low thermal conductivity.7 In other TE materials, the influence of vacancy-type defects on phonon scattering was also found.8−14 Therefore, vacancy-type defects as the phonon scattering center are expected to efficiently reduce lattice thermal conductivity and furthermore increase the efficiency of thermoelectric conversion. However, among the above-mentioned studies, no one can provide confirmative evidence of the reduction of thermal conductivity by vacancies. The reason is simply due to the lack

INTRODUCTION Because of the worldwide energy crisis, thermoelectric conversion as a green energy conversion technique has become a focus of research. By utilizing the thermoelectric conversion technique, most of the waste thermal energy such as the heat from car engines can be converted into electric energy through the Seebeck effect. Usually the performance of thermoelectric materials is evaluated by the dimensionless figure of merit (ZT = S2σT/κ), where S, T, σ, and κ are the Seebeck coefficient, absolute temperature, electrical conductivity, and thermal conductivity, respectively. Efficient thermoelectric (TE) materials should possess a high σ and a relatively low κ; that is, it should be an electron crystal, phonon glass (ECPG) material.1,2 So far, the effective way to increase the figure of merit is the reduction in thermal conductivity κ rather than improvement in power factor (S2σ). The thermal conductivity has two contributions, the electronic thermal conductivity κE and lattice thermal conductivity κL, within which κL plays a major role. Substitution of the host atoms in the thermoelectric materials by doping can sufficiently reduce the lattice thermal conductivity. It has been clearly demonstrated that the atomic mass difference between guest and host atom plays a key role in the reduction of lattice thermal conductivity through phonon scattering,3−5 and the scattering parameter (A) can be expressed as3 A=

Ω0 4πv

x(1 − x) 2

ΔM 2 M © 2014 American Chemical Society

Received: August 9, 2014 Revised: September 4, 2014 Published: September 15, 2014

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20 MPa. Some of the sintered samples then were annealed at four temperatures of 350, 400, 450, and 500 °C for 3 h, respectively. Characterization. The obtained products were characterized by X-ray diffraction (XRD; PANalytical Xpert Pro X-ray Cu Kα) in the 2θ range of 10−80°. Scanning electron microscopy (FEI SIRION) and high-resolution TEM (JEM2010FEF,200 Kv) were also used to observe the microstructure change of the samples during annealing. Positron lifetime spectra were measured using a conventional fast−fast coincidence system with a time resolution of about 220 ps. A 22 Na positron source with intensity of about 20 μCi was used. The positron source was sandwiched between two identical pieces of samples for the positron lifetime measurements. Thermal diffusivity D was measured on a Netzsch LFA 457 laser flash apparatus. Density d of the samples was determined by the Archimedes’ method. The specific heat Cp was measured using a differential scanning calorimeter (DSC Q20, TA Instruments, U.S.), which is about 10% higher than the value estimated by the Dulong−Petit value (∼0.295 J g−1 K−1) at T ≥ 500 K. The resulting thermal conductivity was then calculated using the above measured parameters: thermal diffusivity D, density d, and specific heat Cp by the formula κ = D × Cp × d. The electrical resistivity ρ and Seebeck coefficient S were simultaneously measured using commercial equipment (ZEM3, Ulvac Riko, Inc.) under a low pressure inert gas (He) atmosphere.

of a sensitive method to identify atomic scaled vacancy defects. The routine methods for microstructure characterization such as X-ray diffraction, electron spin resonance, Raman scattering, and Rutherford backscattering can only provide indirect information about defects. Because of this reason, very few experimental works have been reported on the study of defects in thermoelectric materials. There are only a few theoretical calculations on defect properties such as defect formation energy in some TE materials like Mg2Si, Bi2Te3, and CoSb3.15−17 Low dimensional materials such as nanocrystals are found to reduce the lattice thermal conductivity effectively.18 Different origins of strong phonon scattering and thereby significantly low thermal conductivity have been suggested for the nanosized materials, such as the grain boundary and grain shape effect.19−21 The low thermal conductivity comes from the strong phonon scattering by grain boundary. On the other hand, the defects existing in the grain boundary region might be one additional reason for the low thermal conductivity in nanocrystals. However, these two causes for phonon scattering could not be separated from one another because there are always large amounts of defects in the grain boundary region. In addition, there is a lack of appropriate probe for the interfacial defects in nanocrystalline materials. Despite a few theoretical calculations,22 little attention was paid to the effect of interfacial defects on the thermal conductivity. Positron annihilation spectroscopy is a very sensitive probe for atomic-scaled defects in materials.23,24 On the basis of two special properties, having a positive charge and annihilating with electrons, the positron is able to selectively detect vacancytype defects. Positron annihilation parameters at vacancy defects will be different from that of free delocalized states, which makes identification of vacancy defects very simple and clear. This method is particularly appropriate for the study of interfacial defects in nanocrystals.25−29 Generally the diffusion length of positron is longer than the grain radius of nanoparticles, so almost all positrons can reach the surface of grains and will annihilate at vacancy defects in the interface region. Thus, positron is a self-seeking probe for interfacial defects, which enhances the sensitivity of positron to a great extent. In this Article, Bi2Te3 nanocrystals were prepared by hydrothermal method.30 They were sintered under the same condition by a spark plasma sintering (SPS) process and then annealed at different temperatures from 350 to 500 °C in a vacuum. Effect of annealing on vacancy defects in the interface region was studied by positron annihilation measurements. The transportation properties and thermal conductivity of the nanostructured Bi2Te3 materials were also investigated. A good correlation between the lattice thermal conductivity and vacancy-type defects has been observed.

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RESULTS AND DISCUSSION The XRD patterns of Bi2Te3 nanocrystals before and after sintering and annealing are depicted in Figure 1. It can be seen

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EXPERIMENTAL SECTION Synthesis of Bi2Te3 Nanocrystals. Bi2Te3 nanopowders were synthesized by a hydrothermal method. The raw materials were Bi(NO3)3·5H2O (purity 99%), Te (purity 99.99%), EDTA (AR), NaOH (AR), NaBH4 (purity 96%), and distilled water. The raw materials were successively put into an autoclave, and then they were kept at 150 °C for 24 h. After the reaction, the products were filtered and washed with distilled water and ethanol several times and then dried at 110 °C for 6 h under vacuum. The as-grown black powders were treated by a SPS process at 350 °C for 5 min under pressure of

Figure 1. XRD patterns of the Bi2Te3 nanocrystals before and after annealing at different temperatures.

that the diffraction peaks of the as-grown powder and SPS sintered sample are consistent with the standard card for Bi2Te3 with rhombohedral structure (R̅ 3m) (JCPDS card no. 150863). This confirms successful synthesis of Bi2Te3 powders. For all of the annealed samples, the main diffraction peaks are also consistent with the standard card for Bi2Te3. However, a small content of contaminant Bi2TeO5 and Bi2O3 phases is also 22390

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observed. The appearance of contaminant phase may be due to oxidation of these samples during the annealing process. The average grain size of the samples can be estimated from the XRD results by Scherer’s formula: Dhkl = Kλ /β cos θ

(2)

where Dhkl is the average grain size, K is the shape factor (usually taken to be 0.9), λ is the X-ray wavelength of Cu Kα1 radiation, β is the full width at half-maximum (fwhm) of the XRD peak, and θ is the Bragg angle. Standard method to deduct the contribution of instrumental broadening in fwhm has been taken into account. The calculated grain size as a function of the annealing temperature is plotted in Figure 2.

Figure 3. SEM images of Bi2Te3 nanocrystals annealed at different temperatures: (a) SPS treated; (b) 400 °C; (c) 450 °C; and (d) 500 °C.

Figure 2. Grain size of Bi2Te3 nanocrystals annealed at different temperatures.

As shown in Figure 2, the average grain size of the SPS treated sample is estimated to be around 25 nm. After annealing at 350 °C, the average size has only a slight increase by about 5 nm, and it has almost no change up to 500 °C. When samples undergo thermal treatment at 500 °C, the average grain size reaches around 35 nm. Different temperature annealing treatment has no obvious effect on the growth of grain, which may be caused by the surfactant that prevents the growth of grains. SEM images of Bi2Te3 compounds before and after annealing at temperatures of 400, 450, and 500 °C are shown in Figure 3a−d, respectively. All of the sample particles show flake-like structures, and the particle size increases from about 100 nm to a few micrometers with the increase of annealing temperatures. This suggests significant reorganization of nanograins during annealing process, and some grains may agglomerate to form larger particles. Figure 4 shows the HRTEM image of the sintered Bi2Te3 nanocrystals before and after annealing at 500 °C. It can be seen that the sample particles contain many smaller nanograins with different orientations, and the nanograins are highly crystallized. The average grain size is calculated by analysis of dozens of grains in the image. It is clear that the grains grow very slowly during the whole annealing temperature range, which coincides with the XRD results. Positron lifetime spectra were measured for Bi2Te3 samples annealed at different temperatures. All of the spectra can be evaluated into two lifetime components by using the PATFIT program.31 For the unannealed sample, the first lifetime τ1 is around 235 ps, which is mostly the free positron lifetime, possibly with also some contribution from positron lifetime trapped at small vacancies. The second lifetime τ2 is much longer, which is around 380 ps with intensity I2 of about 49%. This is the positron lifetime trapped at open-volume defects

Figure 4. HRTEM images of Bi2Te3 nanocrystals annealed at different temperatures: (a) SPS treated; and (b) 500 °C.

such as vacancies. As the grain size of the sintered samples is about 30 nm, the radius is far less than the diffusion length of positrons, and positron can easily diffuse to the surface of grains. It is thus natural that most positrons will be trapped by vacancy defects in the interface region. To evaluate the size of vacancies by positron lifetime, generally we need to know the positron bulk lifetime in Bi2Te3. However, up to now no one has reported positron annihilation study of Bi2Te3 or related materials, and, on the other hand, the Bi2Te3 single crystal is also difficult to obtain. According to the two-state trapping model, the positron bulk lifetime can be also calculated by the following equation: 1/τb = I1/τ1 + I2/τ2

(3)

However, in nanocrystals, the interface region generally has disordered structure, which contains various defects such as monovacancies, divacancies, or vacancy clusters with different size. These defects cannot be completely separated by positrons, because the positron lifetime for these defects may be close to each other. Therefore, the two lifetime components in our results do not mean that there is only one kind of positron trapping state. It is difficult to obtain the positron bulk lifetime in Bi2Te3 at present. Figure 5 shows the positron lifetime τ1, τ2 and the intensity I2 as a function of the annealing temperature. The average positron lifetime τav is calculated by the following equation and is also plotted in Figure 5: τav = τ1I1 + τ2I2 22391

(4)

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Figure 6. Electrical resistivity (ρ) dependence on temperature of the Bi2Te3 nanocrystals annealed at different temperatures.

Figure 5. Variation of positron lifetime τ1, τ2, intensity I2, and average lifetime τav measured for Bi2Te3 as a function of annealing temperature.

With the increase of annealing temperature, there is no significant change of τ2. This suggests that the defect size has no change during annealing. However, according to Figure 5, the intensity I2 decreases monotonically from 49% to about 20% with increasing annealing temperatures, which indicates lowering of vacancy concentration. This is probably due to the intense movement and realignment of atoms at higher annealing temperatures. The average positron lifetime τav also shows a gradual decrease from 301 to about 273 ps by almost 30 ps after annealing at 500 °C. This is a more reliable parameter to detect any change of the defect properties, because it is insensitive to the decomposition of positron lifetime spectrum. Thus, it further confirms reduction of vacancy concentration after annealing. We can also find that after annealing at 500 °C, the intensity I2 is about 20%. This indicates that some defects still remain after annealing. So the positron bulk lifetime should be shorter than the average lifetime of 273 ps at 500 °C. This means that the ratio of τ2/τb is larger than 1.4. Therefore, we can safely estimate that the defects observed by positrons are primarily vacancy clusters in the grain boundary region. The dependence of electrical resistivity of Bi2Te3 on the annealing temperature is shown in Figure 6. For all of the samples, the electrical resistivity decreases monotonically with increasing temperature, displaying the characteristic of semiconductors. The unannealed sample shows the lowest electrical resistivity relative to annealed samples at the same measuring temperature. The electrical resistivity of annealed Bi2Te3 crystals increases with increasing annealing temperature. When the annealing temperature was above 350 °C, the electrical resistivity became higher than that of the unannealed sample and increased as annealing temperature was raised to 500 °C. The results show that the electrical resistivity of sintered Bi2Te3 is quite sensitive to the annealing temperature. Such similar results have also been observed by others.32 The change of Seebeck coefficient S for Bi2Te3 as a function of annealing temperature is shown in Figure 7. It is found that S

Figure 7. Seebeck coefficient (S) dependence on temperature of the Bi2Te3 nanocrystals annealed at different temperatures.

is negative for almost all of the samples except for that annealed at 500 °C. This suggests that the main carriers initially are electrons for stoichiometric Bi2Te3 crystals. After annealing at 500 °C, the dominant carrier type changes from electron to hole below 250 °C. So all samples exhibit n-type conduction behavior except the Bi2Te3 annealed at 500 °C, which has a cross over from n- to p-type behavior observed at 200 °C, suggesting the presence of a delicate balance between electron and hole transport. It can be noted from Figure 7 that at measuring temperature of 250 °C, all of the Bi2Te3 samples show a negative Seebeck coefficient, and the absolute value of S decreases continuously with increasing annealing temperature. The temperature dependence of thermal conductivity for Bi2Te3 samples is shown in Figure 8. The total thermal conductivity κ as shown in Figure 8a increases monotonously over the entire temperature range with increasing annealing temperatures. To clarify whether the increase of thermal conductivity comes mainly from the electronic thermal conductivity κE or lattice thermal conductivity κL, we employ the relationship κ = κE + κL to separate κL from the total measured thermal conductivity κ. The electronic thermal conductivity was calculated using the Wiedemann−Franz law, κE = LσT, where the Lorenz number, L = 2.45 × 10−8 W Ω K−2. The lattice thermal conductivity of all Bi2Te3 crystals is plotted in Figure 8b. For all of the samples, κL increases with the increasing of annealing temperature, which shows almost the same trend as that of the total thermal conductivity κ. This suggests that the phonon scattering plays a major role in the thermal conductivity. In the whole measuring temperature range of 25−250 °C, the value of κL shows an increase with 22392

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vacancy-type defects in the interface region. The increase of annealing temperature causes a decrease in vacancy concentration. The electrical resistivity ρ of Bi2Te3 increases with increasing annealing temperature, and the Seebeck coefficient S has also some changes for annealed samples. Meanwhile, both the thermal conductivity κ and the lattice thermal conductivity κL of the Bi2Te3 nanocrystals increase with increasing annealing temperature. This strongly suggests the intimate correlation between vacancy-type defects and lattice thermal conductivity κL for Bi2Te3 nanocrystals.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under grant nos. 11275143, 11305117, and the “973 Program” of China under grant no. 2013CB632502.

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Figure 8. Temperature dependence of (a) the total thermal conductivity (κ) and (b) the lattice thermal conductivity (κL) for Bi2Te3 nanocrystals annealed at different temperatures.

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CONCLUSION Bi2Te3 nanomaterials were synthesized by hydrothermal method. The powders were treated with the spark plasma sintering (SPS) process and then annealed at different temperatures. The average grain size of all of the annealed samples was obtained by XRD and HRTEM measurements, which has no obvious change during the whole annealing temperature range. Positron lifetime measurements reveal 22393

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